I am trying to find a server for me to test whatever I need in SQL Server 2000
Now I have located a machine, which has a software application on that server and SQL Server (Which no one uses). Is it possible for me to use this server for SQL purposes? Such as upgrading, reinstall etc.., would such task harm the application that is residing on that server? Or should I get a machine totally dedicated to SQL Server only?
It depends on what else the machine is used for, because you may need to reboot, etc. If the machine is a web server, three tier app server, etc, then it would be a very poor choice for a test machine. If it simply has an application that nobody uses anymore, then it could be a great choice. I suspect that it lies somewhere in between.
The best answer is usually to have a box dedicated for the purpose. That eliminates every "moving piece" that you can from the equation. Sometimes you are constrained by time/space/money so that another solution is "better" for a specific case, but the simplest and safest answer in the long term is usually to have a separate box where you won't disturb another user regardless of what you need to do.